1. Field of the Invention
This invention relates to a sheet delivery apparatus and more particularly to sheet delivery apparatus wherein the pitch diameter of the delivery sprockets is equal to the diameter of the delivery drum.
2. Description of the Prior Art
In the transfer of freshly printed sheets from the printing unit to the delivery apparatus it is essential during delivery that the linear velocity of the sheet remain the same until the trailing edge of the sheet has passed through the nip or tangent point between the blanket and impression cylinder. If the speed of the sheet decreases a curl forms in the trailing edge portion of the sheet and if the speed increases undue tension is exerted on the sheet that results in smearing and streaking of the freshly printed ink.
Conventional sheet delivery apparatus includes a cylindrical sheet supporting means such as a delivery drum or spaced discs on which the sheet is supported as it is transferred from the impression cylinder to the delivery apparatus. A pair of endless chains are reeved about drive sprockets that are nonrotatably mounted on the same shaft that rotatably supports the delivery drum. Gripper bars are secured to the endless chains and extend transversely therebetween. The delivery drum has a recessed portion to receive the gripper bars so that the grippers positioned thereon engage the front edge of the sheet transferred from the impression cylinder. As the front edge of the sheet is engaged by the grippers on the transverse gripper bars the trailing portion of the sheet remains between the printing and blanket cylinder of the printing unit and one side of the trailing edge portion of the sheet is being printed by the ink impression on the blanket cylinder.
It is essential to eliminate smearing of the ink or a curl in the trailing edge of the sheet that the linear speed of the entire sheet remain the same until the trailing edge of the sheet has passed between the nips of the impression and blanket cylinder. Where the linear velocity of the sheet front edge is reduced below the linear velocity of the sheet trailing edge portion the sheet trailing edge portion adheres to the ink on the blanket cylinder because of the ashesive qualities of the ink and forms a curl in the trailing edge portion of the sheet similar to a curl obtained by passing the trailing edge of the sheet over a knife edge. Where the velocity of the trailing edge of the sheet is less than the linear velocity of the sheet front edge portion undue tension is exerted on the sheet that results in smearing of the freshly printed surface. Further, it is highly desirable to provide a smooth, straight path for the sheet after it passes around the delivery drum to eliminate fluttering of the sheet as it is delivered to the stack.
In conventional sheet delivery apparatus the delivery chain sprockets have a pitch diameter substantially smaller than the diameter of the sheet delivery drum and the sheet delivery drum, in turn, because of the different thickness of the sheets that can be printed in the printing press, has a diameter smaller than the diameter of the press impression cylinder. To compensate for sheet thickness, the gripper bar may be adjusted linearly relative to the diameter of the sheet supporting drum to engage sheets of different thickness as they are transferred from the impression cylinder. Where the gripper bar is adjusted beyond the periphery of the delivery drum it has a peripheral velocity greater than the velocity of the delivery drum because of the increased radius of the gripper bar over the radius of the delivery drum. The increased radius increases the speed of the gripper bar as it revolves about the delivery sprockets and exerts an increased tension on the sheet and results in smearing and streaks in the freshly printed ink on the sheet. Remedies, such as gluing foam rubber on parts of the delivery apparatus, do not completely eliminate these deficiencies. Further, the installation of supporting discs, spur gears, helical springs, sand paper strips, foam rubber and the like require considerable space and maintenance.
Where the pitch diameter of the chain sprockets is substantially less than the diameter of the delivery drum the sheet is transferred from the printing unit around the sheet supporting delivery drum at a velocity below the velocity of the sheet transferred by the impression cylinder. This results in the trailing edge of the sheet moving faster than the front edge of the sheet and results in a severe curling of the trailing edge of the sheet after it passes between the nip of the impression and blanket cylinder and before it is supported by the sheet supporting delivery drum.
There is a need for sheet delivery apparatus that maintains the velocity of the sheet as it is transferred from the printing unit to the delivery unit at substantially the same velocity as the sheet is conveyed by the impression cylinder within the printing unit. There is also a need for sheet delivery apparatus that minimizes the change in direction of the sheet as it leaves the circular path around the delivery drum that causes fluttering of the sheet. In the past guides, baffle plates, bands and the like have been used with limited success.
German Patent Application OLS 2,111,049, published and not examined, discloses a sheet delivery apparatus which compensates for the change in the speed of the sheet as it is delivered to the delivery apparatus. This apparatus includes delivery sprockets that have a considerably smaller diameter than the diameter of the delivery drum. The considerable difference in the speed effected by this difference in diameter is compensated by moving a segment of the sprocket to compensate for the different in radius. The disadvantage of this embodiment is that the segment does not revolve and the danger of smearing the wet ink still exists. In another embodiment, a gear is provided in which a double link is periodically shortened and lengthened by a cam to compensate for the existing difference in the speed. This gear only corrects a definite difference in the speed in case of a fixed height of the gripper pad. Where the height of the delivery grippers is changed for different thicknesses of paper, resultant differences in speed of the sheet are encountered. Further, a relatively high play in the gear results from the double link arrangement so that even this correction is not adequate for accurate speed control.
German Patent No. 2,026,355 has also disclosed apparatus in which a stationary drum is provided on the sprocket shaft between the pair of sprockets. This stationary drum has a porous covering through whch compressed air flows inside the drum and escapes through apertures to prevent a smearing of the ink on the printed sheet. However, this apparatus is more suitable for webs of paper which are guided without fluttering on this apparatus than for sheets of paper. Moreover, such apparatus is more expensive than the other known solutions.